Accelerating universe is a term for the idea that our universe is undergoing divergent rapid expansion. In the late 1990s, observations of supernovae (type Ia) produced the unexpected result that the expansion of the universe appears to be accelerating. These observations appear more firm as new data have appeared. This means that not only does the speed with which a distant galaxy recedes from us increase over time, but it increases faster than the linear relationship predicted by the Hubble Law.

If these observations are correct and the acceleration continues, the ultimate result of this trend will be an inability to see any other galaxies any more. A new theory of the end of the Universe based on this finding has been called the Big Rip, although whether the Big Rip occurs depends on the nature of the acceleration and how it changes over time.

Because the energy causing the acceleration of space-time cannot be directly observed, it has been dubbed dark energy. Two theoretical candidates for this energy are a non-zero cosmological constant and a more general repulsive energy known as quintessence. If it's caused by a nonzero cosmological constant, we have a hierarchy problem. Any field would permeate all of space and therefore have the properties of a scalar field much like the one that dominated the expansion of the universe during cosmic inflation. In fact, the dark energy may have actually caused cosmic inflation in the first place.

The observation of an accelerating universe appears to present great problems for Dyson's eternal intelligence. This theory relies on a decelerating universe, which for many years was the dominant model in cosmology, since (in the absence of observational evidence for the existence of dark energy) it was thought that the gravitational attraction of the matter in the universe to itself would act to slow the expansion.

By apparently ruling out a Big Crunch, the accelerating universe also presents great problems for Tipler's Omega Point.

Astronomers estimate that the acceleration of the Universe's expansion began 5 billion years ago. Before that, the expansion was decelerating. The Universe would have been decelerating due to the attractive influence of dark matter. However, it was still expanding, and as it expanded, the attraction of dark matter decreased and the repulsion of dark energy increased. Whenever the Universe doubles in size, dark matter's attraction decreases to one-fourth, but dark energy's repulsion doubles (if energy density remains constant). Thus, the current accelerating phase appears to be dominated by dark energy.

See Ultimate fate of the Universe .